A vaccine to curb heroin dependence. It's been the goal of scientists the world over for decades, but to date, not one has been released to the market. But that could soon change. Mexican scientists have developed a heroin vaccine that has proven to be effective in animal trials, and are now preparing to begin testing in humans. Here's why that's a big deal — and also why it isn't.

This is how it works: the vaccine causes your body to create antibodies that attach to the molecules in heroin that affect your brain. Once these antibodies latch on to their targets, they prevent the drug molecules from accessing certain opioid receptors in your central nervous system, thus extinguishing the neurological reward associated with heroin's use. In animal trials, mice who were given the vaccine demonstrated a huge drop in heroin consumption. By eliminating the rush of pleasure associated with heroin, researchers at Mexico's National Institute of Psychiatry believe they can curb the intensity of the cravings and addiction in humans, as well.

Earlier this month, the Daily Mail reported that the Mexican scientists had made a "breakthrough" with this vaccine, and that a human version could be ready in just five years. Whether or not it's as big a breakthrough as somemedia outlets have made it out to be remains to be seen — but the fact that it's slated to begin human trials means heroin will soon join the ranks of nicotine and cocaine at the forefront of investigations into vaccine-mediated addiction therapy.

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That's why you should be excited. Here's why you should be guardedly optimistic. First of all: five years? Incredibly unlikely. Unless this proves to be a miracle vaccine, the odds of it being available to the public within the next decade are slim. And we have no major indication that this treatment is a miracle-worker. After all, success in animal trials is nothing new in the word of addiction vaccines. In fact, one of the first anti-addiction vaccines that scientists ever developed experimentally was tailored specifically for opiates (like heroin), and was even shown to decrease the self-administration of heroin in rhesus monkeys.

That was in 1974.

Heroin dependence is notoriously difficult to treat, not only because of how addictive the opiate is, but because of the way the body metabolizes it. After it's injected, heroin is broken down into smaller molecules that are capable of generating their own individual psychoactive effects. Drug vaccines work by grabbing ahold of drug molecules and preventing them from accessing the brain, but vaccines can only hang on to these drug molecules if they're specially designed to do so; more heroin metabolites means more molecular targets, and more molecular targets make it harder to create a vaccine that can target them all.

This hangup has been keeping heroin vaccines in the research lab and out of public reach for decades. But that could soon change. Recently, a research team led by Scripps Research Institute's Kim Janda developed the first so-called "dynamic" vaccine strategy — one that is capable of targeting not only heroin, but the molecules it breaks down into. The team's vaccine had the added benefit of being highly specific, meaning it targeted heroin and the molecules it broke down into, while leaving other opioids — like oxycodone — alone.

But it sounds like the drug about to undergo human trials in Mexico may lack these properties. In a recent interview with Reuters, Janda said that based on earlier studies on the drug, the Mexican vaccine "could be reasonably effective but maybe too general, and affect too many different types of opioids as well as heroin." In other words, the vaccine might well function, but probably not without some significant limitations.

For a concise rundown on the current status of therapies for substance abuse disorders (including vaccines, medications currently approved for substance abuse, and potential therapeutic targets), I recommend checking out this commentary, issued earlier this year by the National Institute of Drug Abuse, and co-authored by NIDA Director Nora D. Volkow.